1. Field of the Invention
The present invention relates generally to an apparatus and a method for making fiber reinforced sheet molding compound.
2. Background Art
Filler reinforced thermoset polymers are used in molding applications requiring high strength, light weight, dimensional stability, and corrosion resistance. They are generally molded in matched die sets where heat and pressure are applied to simultaneously cure or crosslink the polymer constituents and shape the desired article. Such articles may replace assemblies of several stamped or cast metal parts.
Many of the thermoset polymer compositions now used in making reinforced plastic components contain crosslinkable polyester resins. However, other viscous resin systems based on polymers, such as vinyl esters, epoxies, phenolics, silicones, phthalates, polyurethanes, etc., are used for their special properties to suit a particular application and may be employed in the practice of this invention.
Sheet molding compound (SMC) is generally defined as a composite molding material which may be made up of unsaturated polyester resin, a low profile thermoplastic polymer agent, an inert filler, a fiber reinforcement, metal oxide maturation aids and processing aids. Generally, the unsaturated polyester resin is the reaction product of polypropylene oxide, phthalic anhydride and maleic anhydride. These constituents are typically provided in solution with styrene monomer. Low profile agents that include such thermoplastic polymers as polymethylmethacrylate, polyvinyl acetate or polybutadienestyrene copolymer rubber are added to improve surface smoothness. The inert filler is usually calcium carbonate, i.e., ground limestone. The fibrous filler is generally chopped fiberglass rovings in lengths of about 0.25 to 2 inches, wherein each roving containing several hundred individual fibers loosely bound together by a starch-like binder. This binder is sometimes referred to as a sizing.
SMC is generally made in a machine specifically designed for such a purpose. A sheet of SMC is produced by applying a thin layer of a low viscosity mixture of resin-styrene monomer and other ingredients onto a continuous, styrene-impermeable, carrier film of polyethylene or other suitable material. The glass fibers are then randomly laid on top of the resin layer. A second layer of the resin mixture is applied over the fiberglass layer and a second protective film is laid on top of it. The resulting sandwich of resin and glass between protective carrier films is then carefully rolled to knead the resin mixture into the glass fibers for proper wet-out. This kneading action must be very gentle as the viscosity of the constituents is relatively low at this time. Application of any substantial compression will cause the resin mixture to ooze out between the edges of the protective films.
The final step of SMC preparation is to coil or fold the just made compound into rolls, and allow them to maturate under controlled temperature conditions (preferably near room temperature, approximately 30° C.) for several days. SMC maturation is generally defined as a process in which the metal oxides react with the free organic acid groups of the polyester constituent to thicken it. Maturation is allowed to proceed to a point where the viscosity of the material has increased to the extent that it can be readily handled for molding and the surface is not too tacky. While metal oxides and hydroxides are the predominant maturation agents, isocyanates are sometimes used as thickeners via a urethane reaction with a polyester component. Alternatively, for an epoxy based SMC, the maturation mechanism may include B-staging, which is a partially completed chemical reaction, resulting in increased molecular weight of the polymer constituents without the occurrence of substantial crosslinking.
The protective films are first stripped from the compound before compression molding maturated SMC. The resin fiber composite, generally 2 to 8 millimeters in thickness, is then cut into suitably sized pieces and stacked into a “charge” of the proper geometry for a specific mold. The charge is placed into a matched metal die mold in a suitable compression molding press. Under typical molding conditions, a mold temperature of about 150° C. is maintained, a molding pressure of about 3.4 to 10.5 mega Pascals is applied, and the article is cured in place for 1 to 3 minutes.
Problems have been encountered in making reliable SMC parts for more rigorous structural applications. Generally SMC has a random fiber orientation which results in substantially isotropic material properties. The orientation of the fibers predominantly affects the strength and the modulus of the material. Current methods of manufacturing SMC do not lend themselves to producing SMC with a specific discontinuous fiber orientation. Accordingly, there is a need for an apparatus and a method for making a directionally oriented, discontinuous fiber reinforced sheet molding compound.